Liquid sample dispensing methods for precisely delivering liquids without crossover

ABSTRACT

A method for reducing carryover of and delivering liquid taken from a source container by a dispensing means to a target container by spinning the target container so that any liquid within the target container is removed from the dispensing means prior to dispensing liquid into the target container.

FIELD OF THE INVENTION

The present invention relates to liquid sample dispensing in which asampling pipette aspirates liquids from a sample or reagent containerand dispenses the aspirated liquid into a reaction vessel. Moreparticularly, the invention relates to a system for precisely deliveringan amount of liquid from a sample or reagent container into a reactiontube and for reducing carry-over of liquid from one reaction tube to thenext, thereby protecting the integrity of the solution within thereaction tube.

BACKGROUND OF THE INVENTION

Various types of tests related to patient diagnosis and therapy can beperformed by analysis of a sample of a patient's infections, bodilyfluids or abscesses. Such samples are typically placed in sample vials,extracted from the vials, combined with various reagents in specialreaction vessels or tubes, incubated, and analyzed to aid in treatmentof the patient. Automated clinical analyzers adapted to perform theseoperations typically handle liquids by aspiration and pressurizeddispensing from the sample vials into a reaction vessel using a samplingprobe or pipette. In general, a sampling pipette is immersed into aliquid held in a suitable container. A partial vacuum is produced in thepipette in an amount sufficient to draw the required amount of liquid upinto the pipette through its nozzle, and the pipette is taken to astation holding a pre-treatment or reaction vessel. At that station,pressure is applied to the interior of the pipette in an amountsufficient to dispense the desired amount of liquid out of the nozzle.The clinical analyzer typically uses a portion or a liquot an aliquot ofthe patient's sample that is aspirated from the vial by a samplingpipette. The entire aspirated aliquot or a portion thereby may then bedispensed from the sampling pipette into a reaction vessel or into asample pre-treatment vessel from which treated sample is lateraspirated. Automated clinical analyzers also typically include reagentpipettes adapted to aspirate reagent from reagent containers and todispense the entire aspirated reagent or a portion thereof into thesample pre-treatment vessel or directly into the reaction vessel.

Conventional pipettes suffer the disadvantage that liquid tends toremain on the exterior surface of the pipette when the pipette iswithdrawn after aspiration. In cases of small volumes of aspiratedliquid, any excess liquid carried on the exterior of the pipette may bea significant volume with respect to or could even exceed the volume ofthe aspirated liquid. Pipettes are designed to accurately dispense apredicted volume of liquid; however, any liquid on the exterior surfaceof the nozzle at the orifice might also be dispensed. Alternatively, thepresence of the liquid on the exterior surface might cause the dispensedquantity of liquid to perfuse up the exterior surface, rather than tomove into a target vessel. In either case, the volume of liquid receivedby the vessel is altered in an unpredictable fashion.

Another disadvantage is that reusable probes used to deliver liquidaliquots from successive containers such as tubes or liquid reagentvessels are a source of intra-sample carryover or contamination.Regardless of application, the sampling pipette and reagent pipette mustalso be thoroughly cleaned and dried between aspirations of differentliquids to avoid carryover contamination.

In the prior art are various solutions to the inter-related carryoverand contamination problems. To prevent cross-contamination betweensamples, the pipette may be provided with a removable and disposable“pipette tip” which is the sole portion of the probe to contact thesample liquid. However, disposable pipettes are costly and over a longperiod of time, become an unexpectedly high item of unwanted expense.Some analyzers include a wiping operation between each aspiration.However, wiping is an extra potential source of contamination, and alsointroduces additional automated mechanisms that lower the throughputrate and increase the expense of an analyzer.

In order to minimize contamination and carry-over between samples, theprobe may be flushed or washed with a diluent liquid such as water. Ithas also been proposed to utilize a separate probe wash sleeve throughwhich a pressurized rinse liquid is flushed (U.S. Pat. No. 4,756,201).In general, a probe wash chamber is utilized including a wash fluidinput into the pipette and a fluid output or exhaust for removing thefluid once the exterior of the pipette has been cleaned. Wash chamberscan leak fluid and also can channel along only one side or a portion ofthe pipette which can leave residue on the pipette exterior.Additionally, if a last drop of wash diluent does not drop off thepipette and is carried back to an aspiration vessel, the droplets dilutethe sample or reagent, introducing unwanted sources of error.

Another technique shown, for example in U.S. Pat. No. 3,266,322,aspirates air through the probe by means of a vacuum pump or theaspirating pump used to withdraw the sample liquid from the samplecontainer. Such aspiration, however, introduces the possibility ofdrawing the unwanted carry-over contaminants deeper into the tubing andapparatus which comprises the sampling system.

U.S. Pat. No. 4,347,875 discloses a “self-cleaning” nozzle for causingliquid remaining behind on the exterior surface of the nozzle toautomatically locate itself other than at the aspirating and dispensingorifice. The nozzle comprises a liquid-confining wall extending about alongitudinal axis and terminating in a liquid-dispensing orifice, and anexterior surface having a portion adjacent to the aperture that isadapted to be immersed into a source of the liquid during aspiration.The wall attracts liquid remaining on the adjacent exterior surfaceafter aspiration to loci spaced from the orifice a distance effective toprevent liquid remaining on the exterior surface from interfering withthe dispensing of the liquid.

U.S. Pat. No. 4,871,682 discloses an air knife positioned to direct astream or blast of air across the tip of a sample probe as it iswithdrawn from a vessel containing a reagent, diluent, and patientsample solution. After the probe is flushed with diluent, the air knifedrives any droplets of diluent fluid off the probe tip into the vesseland thereby prevents contamination or dilution of the sample material inthe sample containers.

U.S. Pat. No. 5,506,142 discloses a wash probe in which the simultaneousintroduction of pressurized air and water creates a turbulent flowincluding the use of a pressurized gas stream of short duration to blowthe residue of the previous sample out of the probe prior to washingwith additional diluent liquid. Also, a waste receptacle is providedwhich uses a filtered air vent and a liquid saturated material aroundthe probe receiving opening to prevent the escape of aerosols from thereceptacle.

U.S. Pat. No. 5,506,142 provides for a probe wash in which thesimultaneous introduction of pressurized air and water creates aturbulent flow including the use of a pressurized gas stream of shortduration to blow the residue of the previous sample out of the probeprior to washing with additional diluent liquid. Also, a wastereceptacle is provided which uses a filtered air vent and a liquidsaturated material around the probe receiving opening to prevent theescape of aerosols from the receptacle.

U.S. Pat. No. 5,536,471 discloses a bubble flushing syringe foraspirating and dispensing fluids through an open-ended tip. The syringecomprises a piston within a bore formed by a cylindrical wall, whereinthe piston forms an annulus with the wall and closed end of the bore,and is capable of reciprocating therein. The syringe further comprisesan annular seal seated in the bore and circumventing the piston toretain fluid when the piston reciprocates therethrough. An inlet fordirecting fluid to the annulus through the wall of the bore and anoutlet for directing fluid from the annulus through the wall of the boreto the open-ended tip are positioned proximal to the annular seal andthe line generally axially therebetween. A drive device is connected tothe piston for reciprocating the piston within the bore. As a result,fluid from the inlet, when connected to a fluid supply, flows around thepiston and through the outlet to the open-ended tip, thereby creating across-flow pattern in the annulus around the piston as it reciprocatesin the bore to flush bubbles through the outlet.

U.S. Pat. No. 5,721,141 discloses a tube washing system including a tubespinning station having a rotatable chuck and a waste chambersurrounding the chuck for capturing and draining tube fluids expelledfrom a spun tube driven in rotation by the chuck. A pipette fordispensing wash water into a tube is located centrally within the chuck.There is also a tube elevating device located beneath the tube spinningstation, the tube elevating device comprising a freely rotatable tubeholder, and lift drive motor provided to vertically move the tube holdertowards and way from the chuck. The tube used in the washing system hasat least one projection provided on its open end which can interlockwith a chuck groove.

U.S. Pat. No. 5,827,744 discloses method for cleaning a liquid sampleprobe in which the probe is positioned within a washing chamber inside awash body and a purging liquid solution is pumped through the probe intothe chamber. A cleaning liquid solution may also be pumped into thechamber around the probe. Either or both liquids are subsequentlyvacuumed from the chamber drawing air through an annular gap between theprobe and the wash body thereby creating a cleaning air flow between theexterior probe surface and the wash body. The cleaning air flow removesall cleaning liquid solution and/or purging liquid solution as the probeis removed from the wash body.

U.S. Pat. No. 6,098,852 discloses a liquid drop dispensing containerwith a dispensing tip that includes a hollow stem. An interior partitionwall within the stem divides the liquid passageway into an upstreamchamber and a downstream chamber. The upstream chamber communicates withthe interior of the container, and the downstream chamber terminates ina liquid drop outlet. A liquid passage is provided in the interiorpartition wall and provides flow communication between the upstreamchamber and the downstream chamber. A unitary head portion extends intothe downstream chamber from the partition wall and defines a roundedliquid impingement surface. Liquid passing from the upstream chamberinto the downstream chamber contacts the rounded impingement surface andpasses dropwise through the liquid drop outlet.

From this discussion of the art state in automated microbiologicalanalyzers, it may be seen that while there has been considerable effortmade toward the problem associated with minimizing contamination andcarry-over between samples, there remains an unmet need for a simplifiedsystem to address the related problems of precisely delivering an amountof liquid from a sample or reagent container to a reaction tube and forreducing carry-over of liquid from one reaction tube to the next.

SUMMARY OF THE INVENTION

The principal object of the invention is to provide a method for using asampling pipette that substantially reduces carryover between samplesaspirated by the pipette and that is simultaneously adapted to preciselydeliver an amount of liquid from a first liquid source container to afirst liquid target container. An important aspect of the invention isto provide a sampling pipette in which any liquid within the targetcontainer is displaced away from the proximate vicinity of the samplingpipette by axially spinning the target container.

By spinning the target container, liquid therein is moved to inner wallsand away from the central portion thereof; consequently, the samplingpipette may be lowered into the target container a sufficient distanceto bring a droplet of liquid at the nozzle of the sampling pipette intocontact with the bottom of the target container. Physically toughing thedroplet with the bottom of the target container releases surface tensionenergy so that the droplet cleanly flows into the target containerwithout contacting the sampling pipette with any liquid spun against thewalls of the target container. Carryover of liquid aspirated anddispensed by the pipette may thereby be minimized between the firstsource container, the first target container and any subsequentlyaccessed source and target containers.

A further aspect of the invention relates to dispensing a droplet fromthe sampling pipette in a first step in which a larger portion of thedroplet is dispensed into a target container, optionally using the abovedescribed method for minimizing carryover of aspirated and dispensedliquids, and the smaller portion is retained within the samplingpipette. After the larger droplet is dispensed, liquid within the targetcontainer is displaced away from the proximate vicinity of the samplingpipette by axially spinning the target container, the sampling pipetteis lowered into the target container to bring the smaller portiondroplet of liquid into contact with the bottom of the target containerso that the entire remaining smaller droplet is dispensed. Sensing toconfirm the “touching-off” of this smaller droplet assures that thetotal volume of liquid dispensed in the two steps is bounded at amaximum by the originally aspirated volume and at a minimum by thevolume of liquid dispensed in the first step alone.

Briefly summarized, the invention provides a method for reducingcarryover of and precisely delivering liquid from a source container tofirst and subsequent target containers by spinning the target containersso that any liquid within the target container is removed from thedispensing means.

BRIEF DESCRIPTION OF THE DRAWINGS

For more complete understanding of the invention reference is made tothe embodiment illustrated in greater detail in the accompanyingdrawings and the following detailed description.

FIG. 1 is a schematic diagram of an automated analyzer in which thepresent invention may be used to advantage;

FIGS. 2-6 schematically illustrate the present invention for reducingcarryover of liquid from a source container to first and subsequenttarget containers; and,

FIGS. 7-11 schematically illustrate an alternate embodiment of thepresent invention for delivering liquid from a source container to anempty target container.

FIGS. 12-17 schematically illustrate an embodiment of the presentinvention for delivering liquid from a source container to a targetcontainer already containing liquid.

DETAILED DESCRIPTION OF THE INVENTION

The method and apparatus of this invention will be described initiallywith particular reference to FIG. 1 of the drawings. FIG. 1 showsschematically the elements of a conventional automatic chemical analyzer10 comprising a sample cup carousel 11 supporting a plurality of opensample tubes 13, a test vessel carousel 14, adapted to hold a pluralityof test vessels 12 and to provide plurality of reagent liquid cartridges20, illustrated as disposed beneath a cut out portion 21 of a lid 22,which covers various thermally controlled compartments. The vesselcarousel 14, preferably in the form of a wheel, has about one hundredseparate open cavities 17 for holding vessels 12, the inner wall of eachcavity having an opening to allow transmission of light. Vessels 12 areseen in FIG. 2 as having a generally cylindrically shape around acentral axis 36, also having an open top 38 and a closed bottom 40. Testvessel carousel 14 is provided with means 34 for rotating selected onesof the test vessels 12 around its central axis 36, the rotating means 34being located proximate selected open cavities 17 holding test vessels12. Reagent cartridges 20 may be, for example, a multi-compartmentcontainer such as those sold under the tradename FLEX® by Dade BehringInc., Deerfield, Ill., and having a number of different reagents withinthe multi-compartments 23. A sample liquid arm 24 and a wash resource 26used to clean a liquid sample aspirating probe 28 described hereinafterare located proximate the sample cup carousel 11 and vessel carousel 14.Sample liquid arm 24 supports sample aspirating probe 28 and is mountedonto a rotatable shaft 27 so that movement of sample liquid arm 24describes an arc intersecting the sample cup carousel 11, test vessels12, and wash resource 26. Sample aspirating probe 28 is adapted, forexample by cooperation with a peristaltic vacuum pump, to aspirate orwithdraw from sample tubes 13 all of or aliquot portions of a patient'sspecimen and to dispense all of or aliquot portions of a patient'sspecimen to be tested by analyzer 10.

In a similar manner, a liquid reagent aspirating probe 25 is rotatablymounted above vessel carousel 16 and is adapted to draw reagent liquidfrom an appropriate compartment 23 of reagent liquid cartridge 20 incooperation with a peristaltic pump vacuum source and to deposit reagentliquid within a predetermined vessel 12 for processing by the chemicalanalyzer 10. Probe 25 optionally comprises an ultrasonic mechanism usedfor aspirating, dispensing and mixing reagents similar to that used inthe DIMENSION® chemical analyzer. Photometic analyzing means, not shown,located beneath the vessel carousel 16 measures light absorbence throughthe vessels 12 at various wavelengths, from which the presence ofanalyte in the sample liquid may be determined. Photometic analyzingmeans, not shown, located beneath the vessel carousel 16 measures lightabsorbence through the vessel 12 at various wavelengths. The photometricanalyzing means is of conventional design and includes a photometer anda source lamp that emits a light beam which passes through various lenshoused in a rotatable detector arm to a photodetector which, beingmounted on the outer-end of the detector arm adjacent the outerperiphery of the vessels 12, rotates about the vessel carousel 16. Thephotodetector relays absorbence readings through the computer where thereadings are converted into concentration units. A conventional computer18 using a microprocessor is used to control the various components ofthe analyzer 10 and to store system parameter changes and test results.The chemical analyzer 10 may be, for example, the DIMENSION® clinicalanalyzer sold by Dade Behring Inc., Deerfield, Ill., or another similaranalyzer commercially available to clinical laboratories.

The present invention adds to analyzer 10 or similar analyzers availableto clinical laboratories a method to precisely deliver an amount ofliquid from a first sample tube 13 into a test vessel 12 and forreducing carryover of liquid within a first test vessel 12 tube toeither a second test vessel 12 or to a second sample tube 13, therebyprotecting the integrity of the solution within test vessels 12 andsample tubes 13. In a more general sense, the present invention providesan method for reducing carryover of and precisely delivering liquid froma source container to a target container by spinning the targetcontainer so that any liquid within the target container is removed fromthe vicinity of the dispensing means. For the purpose of describing theinvention, reference will be made to the aforedescribed sample tube 13as a source container and test vessel 12 as a target container.

FIG. 2 illustrates a test vessel 12 as a target container having anamount of liquid 14 previously disposed therein, for example an amountof reagent taken from a compartment 23 and dispensed therein by liquidreagent aspirating probe 25. Test vessel 12 is shown as being generallysymmetrical around axis 36 for purposes of illustration only. Inpracticing the present invention, a target container need not besymmetrical as long as it may be rotated around a central axis asdescribed next. Prior to the introduction into target test vessel 12 ofadditional liquid taken from a source container sample tube 13 by sampleaspirating probe 28, target test vessel 12 is caused to rotate aroundaxis 36 by a source 34 of rotational motion. Rotational source 34 maycomprise a motor shaft with a tube clamp 25 mounted thereon, a frictionbelt driven by a motor, or other similar mechanisms for rotating targettest vessel 12 around axis 36 at a speed sufficient to cause liquid 14disposed therein to move upwards from the bottom 40 of target testvessel 12 along the inner walls 42 and away from the central portion 46thereof, as illustrated in FIG. 3. By spinning the target container 12,liquid therein is removed from the path of a sample aspirating probe 28,as illustrated in FIG. 4, containing an amount of sample liquid 30aspirated therein. In the present invention, sample aspirating probe 28may have either of a permanent type or of a disposable type aspiratingprobe design.

As a result of liquid within the target container 12 being removed fromthe path of sample aspirating probe 28, aspirating probe 28 may beinserted and lowered into the target container 12 a sufficient distanceto bring a droplet of source liquid 32 formed at the nozzle of theaspirating probe 28 using, for example a peristaltic vacuum pump (notshown), into contact with the bottom 40 of the target container 12, asillustrated in FIG. 5 without the sampling pipette or the droplettouching any of the liquid 14 previously disposed therein. Physicallytoughing the droplet 32 with the bottom 40 of the target container 12releases surface tension energy so that the droplet 32 cleanly flowsinto the target container 12 and not allowing any physical contactbetween the aspirating probe 28 and any liquid spun against the walls 42of the target container 12, as seen in FIG. 6. In FIG. 6, the previousdroplet of liquid 32 formed at the nozzle 33 of aspirating probe 28 hasalso spun against the walls 42 of the target container 12 and admixedwith liquid 14 maintained along the inner walls 42 of the targetcontainer 12 by the centrifugal forces generated by rotational motion ofthe target container 12. This embodiment of the present invention isthus seen to provide a simple method for eliminating contamination ofthe aspirating probe 28 dispensing means by spinning the targetcontainer 12 so that any liquid within the target container 12 isremoved away from the dispensing means when liquid from the sourcecontainer sample tube 15 is dispensed therein by the aspirating probe28. After this dispensing of the initial droplet of liquid 32 into thetarget container 12 by aspirating probe 28 without touching any of theliquid 14 previously disposed therein, the sample liquid remainingwithin aspirating probe 28 may similarly be dispensed into subsequenttarget containers 12 without the aspirating probe 28 touching anyliquids disposed therein. This may be accomplished by axially spinning asubsequent target vessel so that target vessel liquid contained thereinis displaced away from the central portion of the target vessel; forminganother droplet of source liquid at the nozzle of the sampling pipetteand lowering the sampling pipette into the central portion of thesubsequent target vessel a distance sufficient to cause the seconddroplet of source liquid to contact the bottom of the subsequent targetvessel without the sampling pipette or the droplet touching secondtarget vessel liquid disposed therein. Prior to the aspirating probe 28being used to aspirate any additional source liquids taken from anothersource container sample tube 13, the aspirating probe 28 is typicallycleaned by insertion into a conventional wash resource 26. Thisembodiment of the present invention is thereby seen to provide a simplemethod for eliminating carryover of liquid aspirated and dispensed bythe aspirating probe 28 between the source container sample tube 13, thefirst target test vessel 12 and any subsequently accessed targetcontainers 12.

In an alternate embodiment, the present invention may also be useful inovercoming uncertainties associated with dispensing of a known volume ofaspirated liquid that arise due to any portion of the aspirated liquidremaining behind on the exterior surface of the nozzle. This embodimentis illustrated in FIG. 7 in which a known volume of liquid 30 is shownas aspirated into aspirating probe 28 using, for example a preciselymetered peristaltic vacuum pump (not shown), and the aspirating probe 28has been inserted a distance above the bottom 40 of an empty andstationary target vessel 12. The peristaltic vacuum pump is operated todispense a major portion, for example about 98%, of the known volume ofliquid 30 into the stationary target vessel 12., as shown in FIG. 8,leaving a minor droplet portion 32, in this example about 2% of liquid30 within the nozzle 33 of the aspirating probe 28. The peristalticvacuum pump is operated so that the size of the minor portion issufficiently small to ensure that surface tension forces within thedroplet 32 will retain the droplet 32 at the nozzle 33 of the aspiratingprobe 28.

As described before, target test vessel 12 is next caused to rotatearound axis 36 by a source 34 of rotational motion at a speed sufficientto cause liquid 30 dispensed therein to move upwards from the bottom 40of target test vessel 12 along the inner walls 42 and away from thecentral portion 46 thereof, as illustrated in FIG. 9. After thedispensed major portion of liquid 30 into the target container 12 ismoved upwards from the bottom 40 to the inner walls 42 by centrifugalforces generated by the rotational motion of the target test vessel 12,aspirating probe 28 may be lowered into the target vessel 12 to bringminor droplet portion 32 at the nozzle of the aspirating probe 28 intocontact with the bottom 40 of the rotating target container 12, asillustrated in FIG. 10, thereby releasing surface tension energy so thatthe droplet 32 cleanly flows into the target vessel minimizing theamount of any aspirated liquid remaining behind on the exterior surfaceof the nozzle. The minor droplet portion 32 then admixes with the majorportion of liquid 30 along the inner walls 42 of the target vessel 12,illustrated in FIG. 11.

In this embodiment, the present invention ensures that the total amountof liquid 30 dispensed into the target vessel 12 is less than the totalvolume of liquid 30 originally aspirated into aspirating probe 28 and,at the same time, is greater than the major portion of the known volumeof liquid 30. It should be noted by the reader that while this alternateembodiment may be practiced when the target vessel 12 is originallyempty, as described above, in an instance that the target vessel 12originally contains a liquid 14, like shown in FIG. 2, then as describedin conjunction with FIGS. 3 and 4, the target test vessel 12 may berotated to cause liquid 14 disposed therein to move from the bottom 40of target test vessel 12 along the inner walls 42 and away from thecentral portion 46 thereof, as illustrated in FIG. 3 and away from thepath of the sample aspirating probe 28, as illustrated in FIG. 4.

Such an embodiment is illustrated in FIGS. 12-17 where, beginning withFIG. 12, a target vessel 12 contains target fluid 30 therein a and aknown amount of source liquid 32 has been aspirated from a source vesselinto an aspirating probe 28. The target vessel is spun around its axis36, FIG. 13, so that the target liquid 30 contained therein is displacedaway from the central portion 46 and the bottom portion 40 (see FIGS. 2and 3) of the target vessel 12.

Next, the sampling pipette 28 is lowered into the central portion 46 ofthe target vessel 12 a distance sufficient to cause a major portion 50of the volume of source liquid 32 to contact the bottom of the targetvessel 12, FIG. 14, without the major portion 12 touching target liquid30 disposed therein, so that the major portion 50 of source liquid 32 isspun off from the sampling pipette 28 into the target vessel 12. Thetarget vessel 12 continues to spin so that the target liquid 30 and themajor portion 50 of source liquid 32 contained therein are displacedaway from the central portion of the target vessel, FIG. 15, and admixedtogether, illustrated as mixture 51.

Subsequently, a minor droplet 52 of the remaining portion of sourceliquid 32 is formed at the nozzle of the sampling pipette 28 and thesampling pipette 28 is again lowered into the central portion 46 of thetarget vessel 12 a distance sufficient to cause the remaining minordroplet portion 52 of source liquid 32 to contact the bottom of thetarget vessel 12 without the remaining minor droplet portion 52 touchingliquid mixture 51 disposed therein, FIG. 16, so that the remaining minordroplet portion 52 of source liquid is spun off from the samplingpipette into the target vessel 12, FIG. 17, and admixed together withmixture 51, illustrated as mixture 53.

It is to be understood that the embodiments of the invention disclosedherein are illustrative of the principles of the invention and thatother modifications may be employed which are still within the scope ofthe invention. For example, if small amounts of liquid are involved,rather than spinning the target vessel to remove liquid therein from thepath of the aspirating probe, the target vessel may be inclined at anangle to remove liquid from the bottom portion of the target vessel sothat the aspirating probe may be lowered into the target container tobring the droplet of liquid at the nozzle of the aspirating probe intocontact with the bottom of the target container without touching any ofthe liquid previously disposed therein. Accordingly, the presentinvention is not limited to those embodiments precisely shown anddescribed in the specification but only by the following claims.

1. A method for reducing carryover of the first target vessel liquidcontained in a first target vessel and a sampling pipette containingsource liquid, the method comprising: spinning the first target vesselso that the first target vessel liquid contained therein is displacedaway from the central portion of the target vessel; forming a firstdroplet of source liquid at the nozzle of the sampling pipette; loweringthe sampling pipette into the central portion of the target vessel adistance sufficient to cause the first droplet of source liquid tocontact the bottom of the first target vessel without the samplingpipette or the droplet touching first target vessel liquid disposedtherein, so that the first droplet of source liquid is spun off from thesampling pipette into the target vessel.
 2. The method of claim 1further comprising: spinning a second target vessel so that secondtarget vessel liquid contained therein is displaced away from thecentral portion of the second target vessel; forming a second droplet ofsource liquid at the nozzle of the sampling pipette; lowering thesampling pipette into the central portion of the second target vessel adistance sufficient to cause the second droplet of source liquid tocontact the bottom of the second target vessel without the samplingpipette or the droplet touching second target vessel liquid disposedtherein, so that the second droplet of source liquid is spun off fromthe sampling pipette into the second target vessel.
 3. The method ofclaim 1 wherein the target vessel is axially symetrical symmetrical. 4.The method of claim 1 wherein the sampling pipette is either a permanentor disposable sampling pipette.
 5. A method for reducing carryover offirst target vessel liquid contained in a first target vessel and asampling pipette containing source liquid, the method comprising:inclining the first target vessel so that first target vessel liquidcontained therein is displaced away from the central portion of thetarget vessel; forming a first droplet of source liquid at the nozzle ofthe sampling pipette; lowering the sampling pipette into the centralportion of the target vessel a distance sufficient to cause the firstdroplet of source liquid to contact the bottom of the first targetvessel without the sampling pipette or the droplet touching first targetvessel liquid disposed therein, so that the first droplet of sourceliquid is spun off from the sampling pipette into the target vessel. 6.The method of claim 5 further comprising: inclining a second targetvessel so that second target vessel liquid contained therein isdisplaced away from the central portion of the second target vessel;forming a second droplet of source liquid at the nozzle of the samplingpipette; lowering the sampling pipette into the central portion of thesecond target vessel a distance sufficient to cause the second dropletof source liquid to contact the bottom of the second target vesselwithout the sampling pipette or the droplet touching second targetvessel liquid disposed therein, so that the second droplet of sourceliquid is spun from the sampling pipette into the target vessel.
 7. Themethod of claim 5 wherein the target vessel is axially symetricalsymmetrical.
 8. The method of claim 5 wherein the sampling pipette iseither a permanent or disposable sampling pipette.
 9. A method fordelivering an amount of liquid from a source vessel into a targetvessel, the method comprising: aspirating a known volume of liquid fromthe source container into an aspirating probe; dispensing a majorportion of the volume of liquid into the target vessel while the targetvessel is stationary, at the same time leaving a minor portion of liquidwithin the aspirating probe; spinning the target vessel so that themajor portion of liquid contained therein is displaced away from thecentral portion of the target vessel; forming a droplet of the minorportion of source liquid at the nozzle of the sampling pipette; loweringthe sampling pipette into the central portion of the target vessel adistance sufficient to cause the droplet of source liquid to contact thebottom of the target vessel without the droplet touching liquid disposedtherein, so that the droplet of source liquid is spun off from thesampling pipette into the target vessel.
 10. The method of claim 9wherein the target vessel is axially symetrical symmetrical.
 11. Themethod of claim 9 wherein the sampling pipette is either a permanent ordisposable sampling pipette.
 12. A method for delivering an amount ofsource liquid from a source vessel into a target vessel containingtarget liquid therein, the method comprising: aspirating a known volumeof source liquid from the source container into an aspirating probe;spinning the target vessel so that target liquid contained therein isdisplaced away from the central portion of the target vessel; loweringthe sampling pipette into the central portion of the target vessel adistance sufficient to cause a major portion of the volume of sourceliquid to contact the bottom of the target vessel without the majorportion touching target liquid disposed therein, so that the majorportion of source liquid is spun off from the sampling pipette into thetarget vessel; continuing to spin the target vessel so that all liquidcontained therein is displaced away from the central portion of thetarget vessel; forming a droplet of the remaining droplet portion ofsource liquid at the nozzle of the sampling pipette; lowering thesampling pipette into the central portion of the target vessel adistance sufficient to cause the remaining droplet portion of sourceliquid to contact the bottom of the target vessel without the remainingdroplet portion touching liquid disposed therein, so that the remainingdroplet portion of source liquid is spun off from the sampling pipetteinto the target vessel.
 13. The method of claim 12 wherein the targetvessel is axially symmetrical.
 14. The method of claim 13 wherein thesampling pipette is either a permanent or disposable sampling pipette.15. The method of claim 1, wherein the sampling pipette is lowered intothe central portion of the target vessel without the sampling pipette orthe droplet touching first target vessel liquid disposed therein. 16.The method of claim 2, wherein the sampling pipette is lowered into thecentral portion of the target vessel without the sampling pipette or thedroplet touching second target vessel liquid disposed therein.
 17. Themethod of claim 5, wherein the sampling pipette is lowered into thecentral portion of the target vessel without the sampling pipette or thedroplet touching first target vessel liquid disposed therein.
 18. Themethod of claim 6, wherein the sampling pipette is lowered into thecentral portion of the target vessel without the sampling pipette or thedroplet touching second target vessel liquid disposed therein.
 19. Themethod of claim 9, wherein the sampling pipette is lowered into thecentral portion of the target vessel without the droplet touching liquiddisposed therein.
 20. The method of claim 12, wherein the samplingpipette is lowered into the central portion of the target vessel adistance sufficient to cause a major portion of the volume of sourceliquid to contact the bottom of the target vessel without the majorportion touching target liquid disposed therein.
 21. The method of claim12, wherein the sampling pipette is lowered into the central portion ofthe target vessel a distance sufficient to cause the remaining dropletportion of source liquid to contact the bottom of the target vesselwithout the remaining droplet portion touching liquid disposed therein.